1. Field of the Invention
The present invention provides a filtering device for filtering a signal reproduced from an optical data storage medium and a method for the same. More particularly, the present invention provides a device for suppressing variation of the DC level of a quad sum signal or a push-pull signal.
2. Description of the Prior Art
Optical data storing devices use the optical method to write digital data into optical data storage medium. In the prior art, optical storage medium are usually some type of disks. The method of storing optical data is to sequentially store digital data onto the data tracks of the disk. To reproduce the digital data of the disk, a pickup head (containing a light detector) emits a laser light onto the optical disk and then receives the reflected laser light from the optical disk. The received laser light is transferred into electrical signals.
In the prior art, the data is recorded sequentially in the disk from inside to outside in a plural concentric circle format or spiral format, and different reflection results are used to differentiate the digital data into 0 or 1. When data is recorded on the disk, it forms a plurality of circular tracks. When the optical data reproducing device reproduces data from the disk, the pickup head follows the tracks to emit laser light onto the disk, and reflected light is generated by the reflection of the laser light according to the different reflection rate produced during recording data on the disk. The light detector of the pickup head receives the reflected laser light from the disk, which is then transferred into electrical signals.
FIG. 1 illustrates a structure of the optical data storage medium and the waveform of the corresponding quad sum signal and push-pull signal. Generally, read/write optical disks of digital audio and video (DVD-RAM) record data in two kinds of storing tracks, land track 81a and groove track 81b. Both kinds of tracks can be used to store data in a set of sectors 82. Each sector 82 includes a header field 82a for carrying address data and a data field 82b for carrying user data. The period (tA,tB) corresponds to the data field of the previous sector. The period (tB,tC) corresponds to the header field of the current sector. The period (tC,tD) corresponds to the data field of the current sector. The address data and user data are formed with pits. Pits in the data field 82b are formed along the central datum within a track. The header field 82a composed of two headers, each physically offset from the other relative to a central datum within a track. The laser light emitted from the pickup head is typically focusing on the central datum of the track. A light detector, located in the pickup head, then receives the reflected laser light from the optical data storage medium, splits the reflected laser light into four quadrants and generates at least four light detection signals denoted by Sa, Sb, Sc, and Sd. Based on these light detection signals, a quad sum signal (Ssum) and a push-pull signal (Spp) is obtained. The Ssum=Sa+Sb+Sc+Sd represents the sum up brightness of the reflected light, and Spp=Sa+Sd−(Sb+Sc) represents the brightness difference between two parts of reflected divided by the centerline. As known by people skilled in the art, Ssum and Spp are important signals while reproducing user data and controlling the DVD-RAM drive. However, the pits have different locations in the header field 82a and data field 82b which leads to different brightness of the reflected light. The different brightness of the reflected light further cause different DC level of signals Ssum and Spp corresponding to header field 82a and data field 82b. The optical data reproducing device should able to adequately remove the difference of the DC level of the signals Ssum and Spp between the header field 82a and the data field 82b so as to correctly reproduce the user data.